The systems of taking and reproducing images with depth developed up to now can be divided into two major groups, namely the most modern ones, developed since 1947, based on the formation of images due to the interference of beams of coherent light, which are called holographic systems, and the oldest, which do not record by interference of waves, which are known as non-holographic systems.
Among the latter, stereoscopic and three-dimensional systems are distinguished from each other. The term stereoscopic is used for systems in which two distinct images, one for each eye, are used in the reproduction. The term three-dimensional is used to distinguish systems which use a greater number of taken and reproduced images, allowing observation within a wide viewing angle, without inconveniencing the observers by placing optical filters or any other contrivance before them.
The technique of holography is based on photography by reconstruction of wavefronts. These systems require coherence of the light sources for image-taking and reproduction. Both the objects which are to be recorded as well as the images which are to be reproduced need to be illuminated with coherent light only. This has hindered the commercializing of systems using this process which are capable of making photographs of distant objects which, like the moon, cannot be lit up with a coherent beam. It is found impossible to photograph sunsets or reflections of the sun or moon on the sea, landscapes etc. Finally, as observation through transparency is necessary, the size of the reproduced image is limited.
In the stereoscopic systems, the photograph is taken through two objectives which are separated from one another by a distance approximately equal to the average value of the distance between human eyes.
For this stereoscopic photography, special systems of lenses have been developed which are suitable for attachment to conventional cameras, such as the Fazekas camera, which is described in U.S. Pat. No. 4,525,045.
There are stereoscopic systems in which the bringing of a different image to each eye is obtained by processes which are not suitable for projection. They include those which place an optical system, such as the Brewster prisms and Wheatstone flat mirrors, between the observers and reproduced image [Norling, J. A., The Stereoscopic Art . . . A. Reprint. J. Smpt 60, No. 3, 286-308 (March 1953)], or the Kempf concave mirror [U.S. Pat. No. 4,623,223].
The stereoscopic systems suitable for projection differ greatly depending on the process used to bring the image taken up by the left lens to the left eye and that taken up by the right lens to the right eye. The best known and most widely used in stereoscopic projections with movement are those which employ colored, polarized filters or shuttering.
The main limitation of the stereoscopic systems used in projection is that they necessarily inconvenience the observer by placing optical filters or a shuttering mechanism in front of him.
Among the three-dimensional systems of reproduction using ordinary light which have been developed up to the present, there are some which are capable of showing the reproduced image on the right or left side when the observer moves to the left or right or vice versa.
Most of these three-dimensional reproduction devices employ a diffusion surface on which the various images are printed, projected, generated, amplified or simply transmitted. Typical printing systems are ones which use the photographic material itself as a diffusion surface with projection onto an opaque or translucent surface in cinematography or projected television; typical generation systems are those in which the diffusion surface is the cathode ray tube itself; and typical transmission systems are those which employ light conductors or amplifiers.
It is important to emphasize one essential characteristic common to any diffusion surface which greatly affects the design of all devices for three-dimensional reproduction which use this type of surface.
This essential characteristic is that: "Any point of the diffusion surface is converted into a center transmitting light photons in all directions."
As a consequence, any observer, whatever his position, will see the whole image reproduced on the diffusion surface.
If two or more images are reproduced at the same time on the same point of the diffusion surface, the photons coming from the different images appear mixed together, whatever their direction.
For this reason, distinguishing the different images reproduced on the diffusion surface is achieved by reserving a different place for each of them, that is by means of "scalar image differentiation".
All systems which contain a diffusion screen succeed, by different methods, in reserving a different portion on it for each image. This position is usually a vertical band of very small width.
In the systems for the reproduction of stationary photographic images it is on the photographic material itself, which acts as diffusion surface, that the images appear divided into fine vertical stripes. The element entrusted with dividing the images into fine stripes is normally a sheet of cylindrical lenses.
Among the systems which use this technique mention may be made of the following:
U.S. Pat. No. 1,918,705 to Ives which describes a process for obtaining three-dimensional images on photographic material.
Glenn's U.S. Pat. No. 3,482,913 which describes a method and the apparatus needed to compose three-dimensional photographs.
Wah-Lo's U.S. Pat. No. 4,086,585 which describes a system and a camera for controlling the field depth in three-dimensional photography.
In the systems for the reproduction of moving images by projection, the diffusion surface consists of an opaque material if the projection is a front projection or of a translucent material if the projection is a rear projection. In all cases, the images appear on this surface divided into fine vertical stripes.
Among the systems which employ this technique mention may be made of:
U.S. Pat. No. 1,883,290 to Ives which describes a method of front projection on an opaque screen in which the element which divides the image into fine vertical stripes is the same sheet of vertical cylindrical lenses through which observation is effected and another method of rear projection onto a translucent surface in which the element which divides the images into fine vertical stripes is also a sheet of vertical cylinders used in the method of copying different films onto a single one. In the first case, it will be necessary to eliminate the brightness generated by the mirror image of the projectors on the cylinder sheet and in the second an adjustment of high precision is required in order to position the image stripes on each cylinder.
U.S. Pat. No. 4,078,854 to Yano describes two methods of three-dimensional reproduction by rear projection. In the first, corresponding to FIGS. 1, 2, 3 and 4 of the patent, the diffusion screen, which is made of translucent material, appears between two sheets of vertical cylindrical lenses. One of these sheets of cylindrical lenses has the task of dividing the image into fine vertical stripes on the diffusion surface. The second method of this patent, which replaces the diffusion surface by a sheet of horizontal cylindrical lenses will be discussed further below.
U.S. Pat. No. 4,737,840 to Morishita describes a method which is based on rear projection through a vertical opaque grid located in front of the diffusion surface. In the diffusion surface each image always appears at a different place, in a different vertical stripe.
Other processes of reproducing moving images exist in which the diffusion surface is formed by the ends of light conductors, as described in U.S. Pat. No. 4,571,616 to Haisma, in which each image also appears within a different vertical stripe. In this case, the images are positioned after being guided through light conductors.
In all these cases, the viewing is effected through an optical screen of vertical cylindrical lenses the focal lines of which are contained in a plane in which the diffusion surface is situated.
Below there is first given a critical examination of the three-dimensional horizontal parallax reproduction systems described above.
The factors to be taken into account in the comparison of the different systems are:
The orthoscopic viewing angle, the quality of the image reproduced, and the cost resulting from the complexity of manufacture.
The maximum viewing angle is limited by the aperture of the vertical cylinder, the ratio between the width thereof and its focal length; if this angle is exceeded, observation takes place on an image line corresponding to the adjacent cylinder, producing the undesirable pseudoscopic effect, that is to say, inverted depth.
Haisma, in his aforementioned patent, (see page 1-65) points out the importance of this problem.
If the set of stripes corresponding to each cylinder occupies the width of the latter, the maximum viewing angle without pseudoscopy is expressed by: ##EQU1## which, for ordinary materials, the indices of refraction of which vary around 1.5, has an approximate value of 54.degree., which is clearly insufficient in many cases.
The preservation of this angle on the width of the entire screen requires precise correspondence between each cylinder and its image (group of stripes). This correspondence is difficult to achieve when the lenticular sheet charged with generating the image divided into fine vertical stripes is not the same as that used in the observation of the image. This lack of correspondence is a problem to be taken into account in the photographic reproduction systems and in those other rear projection systems in which, such as that used by Ives 290, the division of the image into fine vertical stripes is effected in a process different from that of projection. This difficulty is foreseen by Ives 290 himself although he does not propose any method of solving it (see page 3, 103-106).
Since the orthoscopic viewing angle is a function of the ratio between the width of the image and its focal length, in order to increase this angle two procedures may be employed: either increase the width of the image corresponding to each cylinder or decrease the focal length of the cylinder with respect to its width, using materials of very high indices of refraction (close to 2).
Both methods are mentioned in Ives 290.
The first, increase in the size of the image reproduced, can be noted on page 3. In this method, a loss of quality upon reproduction results, due to the fact that the distance between the axes of the cylinders is greater than the diameter thereof, dark vertical lines therefore appearing between cylinders; see Ives 290, page 3, 65-75. The enormous complexity of manufacture of this lenticular sheet is obvious.
The second, based on the relative decrease of the focal length by increase of the index of refraction, leads to the need of placing opaque sheets between cylinders, substantially complicating manufacture (see Ives 290, page 4, 45-50).
In both cases, the complex section of these cylinders recommends front projection and, as a result, an undesirable brightness appears on the lenticular sheet caused by the specular vision of the projectors. This new difficulty makes it necessary to project onto a suitably inclined vertical sheet facing the projectors and observers (see Ives 290, page 4, 60-65).
The quality of the image is limited by the transverse dimension of the cylindrical lens, which, in its turn, is limited by that of the vertical band of the image.
It is to be taken into account that the width of each vertical image band must be as many times less than the size of the cylinder as the number of images reproduced. For this reason, the size of the cylinder is limited by the size of the image, which, in its turn, is less than that of said cylinder.
The condition for a stripe of width "d" not being perceptible for a healthy eye is that ##EQU2##
For example:
0.3 mm for a distance of 1 m, and PA1 0.08 mm for a distance of 0.25 m. PA1 Any viewer, whatever his position, will see a single point of the projected image. This point is the intersection with the transparent surface of the line which joins the optical center of the projector objective to the optical center of the viewer. For each position of observation there will correspond a separate image or point. PA1 If two or more images are projected at the same time from different positions in space onto the transparent surface, the photons coming from the different projections will retain their direction after passing through it. The different images can be distinguished because the photons of each emerge from this transparent surface at a different angle; that is to say "Angular Image Differentiation" can be used. PA1 (A) The orthoscopic viewing angle can be made as large as desired, it depending only on the number of projectors, the distance between them, and the projection distance. PA1 (B) The size or width of the cylinders is not limited by the number of images and can be designed as small as desired, so that the quality of the image is only limited by the conditions of manufacture of these cylinders. PA1 (C) When the viewer leaves the field of vision, no pseudoscopy takes place. PA1 (D) It is not necessary to create a complex means for dividing the projected images into ordered and interlaced vertical stripes, nor is there required the collaboration of other convergent optical systems in addition to the lenticular plane, the rear projection requires no adjustments in precision and, finally, the system of the invention is easier to manufacture and simpler to implement whatever the size of the image reproduced. PA1 (E) The images perceived by each eye of the viewer are different, regardless of his location. PA1 (F) The integral reproduction systems are easy to manufacture. PA1 (G) Front projection is achieved very easily by replacing one of the sheets of lenses by another sheet of mirrors.
If 10 images are used, the width of each image stripe must be 0.03 and 0.008 mm respectively. These values are on the order of only 15 times greater than the wavelength of visible light. If a number of images greater than 10 were used the situation would, logically, become worse. The difficulties in manufacture are obvious and, therefore, the price of the commercial product is high. In systems in which, like Haisma's, the image is positioned through optical conductors, this difficulty may be incapable of solution.
It is important to point out that the inventors of systems based on the scalar differentiation of images, who have tried to provide their system with a large orthoscopic viewing angle, have had to solve the problem of designing cylindrical elements with a large aperture.
This is the reason why, in these systems, the orthoscopic viewing angle coincides with the aperture angle of the vertical cylinders through which the viewing is effected.
For this reason, the systems based on the scalar differentiation of images of high value of orthoscopic viewing angle give rise to designs of vertical cylinders which are very expensive or impossible to construct.
Furthermore, a high orthoscopic viewing angle, with the need of continuity and great depth in the reproduction, requires a large number of images. As has been pointed out, a large number of images, in a scalar differentiation system, requires some cylinders also of high transverse size, since each cylinder must house as many stripes as images, and these stripes cannot be made indefinitely small. Therefore, the size of the cylinders is conditioned and the quality of the reproduction may be deficient.
These reasons explain why these systems have not been successfully marketed, not even in cinematography with small projection screens.
Secondly, within this general technique, there are included the integral reproduction systems. This is the name given to systems capable of reproducing horizontal and vertical parallax simultaneously.
The invention is that of Lippmann, the famous French optician in 1908 (Lippmann, M. G., Epreuves Rversibles Donnant la Sensation du Relief. J. Phys. 7, 4th Series, 821-825 (Nov-1908)).
The basis of integral photography is to prepare fly's eyes lens sheet, of glass or plastic, with a tremendous number of spherical plano-convex lenses (for example 10,000).
One example of integral reproduction is Ando's U.S. Pat. No. 3,852,524.
Ando, at no time, mentions the number of images taken nor the width of the band required for their transmission; he simply says that they are multiple and that a carrier of very high frequency must be used.
In fact, this process of image taking and reproduction requires the handling of an enormous amount of information, because 2-dimensional image is received behind every plano-convex lens.
In order to make the system work, the number of plano-convex lenses used, both for the reproducing and the taking of images, must be on the order of thousands.
Apart from these difficulties and the use of spherical optical screens, the reproduction is always carried out in all the forms described in his patent through a diffusion surface with all the drawbacks which this use entails.
Haisma, in his aforementioned U.S. Pat. No. 4,571,616 describes an integral relief system based on taking the image with conventional cameras forming a square mosaic. He gives as an example a number of 9 cameras arranged in 3 columns of 3 cameras each.
The reproduction is continued by positioning nine different sections of images behind each spherical lens, at the rate of one section for each image taken. The adjustment is achieved by appropriately positioning the optical conductors by mechanical means. While we have previously seen the complexity of manufacture brought about by the positioning of image stripes behind each cylinder, the problem here is much more serious, since it involves positioning n.sup.2 squares of images behind each spherical microlens.
In addition, in the system described by Haisma a diffusion surface is used, in this case the ends of optical conductors.
The above-mentioned drawbacks of the horizontal parallax reproduction systems also appear here not only in the reproduction of the horizontal parallax but also in the reproduction of the vertical parallax and they have prevented the successful marketing of this system.
Certain other fields of application also exist, as in robotics, where also, as in the aforementioned Ando patent, optical screens of spherical lenses are used; see for example U.S. Pat. No. 4,410,804 to Stauffer. His purpose, however, is to obtain data on the distance of the objects and their size, and never three-dimensional reproduction with vertical and horizontal parallaxes of images.
The only system of which knowledge is had which does not employ diffusor surfaces is that described by Yano.
Yano, in the second part of his U.S. Pat. No. 4,078,854, replaces the diffusion surface by a sheet of horizontal cylindrical lenses, but does not do so in order to design a new system based on the angular differentiation of images, but rather as a mere variant of what was described in the first part and based on the scalar differentiation on a diffusion surface of translucent material.
In fact, the system is referred to in that patent as a stereoscopic reproduction system, having a very small number of images reproduced (see page 1, 10-13) which at most can be five (see page 5, 30-32) with a wide viewing margin, but, as recognized in that patent (see page 3, 32-36), there are viewers who will see the same image with both eyes and for whom it is necessary to provide a sensation of depth by other means.
For this, this last-mentioned device has two elements, a convergent optical system which conditions and makes difficult the projection of images of large size and a screen composed of two sheets of cylindrical lenses; the aperture of the cylinders has a concrete and fixed value independent of the distance between objectives-projectors and the projection distance; which makes it obvious that its design is not based on the angular differentiation of images.
Accordingly, this last-mentioned system described in the second part of Yano's Patent is a mere variant of what is described in the first part, which is based on the scalar differentiation of images and like all the others, with various drawbacks.
Finally, it should be noted that the above systems were designed in order to cause an image taken in its entirety by a single camera to reach each eye. There is concerned the achieving of three-dimensional vision by causing each eye to see an image taken by a different camera and therefore located at a different place.
This concept is described in greater or lesser length by the previous inventors; see for example, Ives 290 page 4, 10-25, Ives 705 page 1, 95-100 and page 2, 0-2 Glen page 1, 65-70, Haisma page 1, 24-29, Yano page 1, 14-19 and page 2, 29-32.